Electrical and Energy

Worldwide, many different mains power systems are found for the operation of household and light commercial electrical appliances and lighting. The different systems are primarily characterized by their:

but also by their:

All these parameters vary among regions. The voltages are generally in the range 100–240 V (always expressed as root-mean-square voltage). The two commonly used frequencies are 50 Hz and 60 Hz.

Some territories use standards different from those of the countries they belong to (such as Hong Kong in China, similar voltage but different AC power plugs and sockets). Foreign enclaves, such as large industrial plants or overseas military bases, may have a different standard voltage and frequency from the surrounding areas. Some city areas may use standards different from that of the surrounding countryside. Regions in an effective state of anarchy may have no central electrical authority, with electric power provided by incompatible private sources.

Many other combinations of voltage and utility frequency, including direct current, were formerly used, with frequencies between 25 Hz and 133 Hz and voltages from 100 to 250 V. The modern standard combinations of 230 V/50 Hz and 120 V/60 Hz did not apply in the first few decades of the 20th century and are still not universal.

Industrial plants with polyphase power systems will have different, higher voltages installed for large equipment (and different sockets and plugs), but the common voltages listed here would still be found for lighting and portable equipment.

In most countries, household power is single-phase electric power, with two or three wired contacts at each outlet.

Various earthing systems are used to ensure that the ground and neutral wires have the correct voltages, to prevent shocks when touching grounded objects.

In some installations, there may be two live conductors which carry alternating currents in a three wire single-phase circuit.

Small portable electrical equipment is connected to the power supply through flexible cables terminated in a plug, which is then inserted into a fixed receptacle (socket). Larger household electrical equipment and industrial equipment may be permanently wired to the fixed wiring of the building. For example, in North American homes a window-mounted self-contained air conditioner unit would be connected to a wall plug, whereas the central air conditioning for a whole home would be permanently wired. Larger plug and socket combinations are used for industrial equipment carrying larger currents, higher voltages, or three phase electric power.

Circuit breakers and fuses are used to detect short circuits between the live and neutral wires, or the drawing of more current than the wires are rated to handle to prevent overheating and fire. These protective devices are usually mounted in a central panel in a building, but some wiring systems also provide an over current protection device at the socket or within the plug.

Residual-current devices, also known as ground-fault circuit interrupters and appliance leakage current interrupters, are used to detect ground faults - leakage of current to somewhere other than the neutral and live wires (like the ground wire or a person). When a ground fault is detected, the device quickly cuts off the circuit.

A distinction should be made between the voltage at the point of supply (nominal system voltage) and the voltage rating of the equipment (utilization voltage). Typically the utilization voltage is 3% to 5% lower than the nominal system voltage; for example, a nominal 208 V supply system will be connected to motors with "200 V" on their nameplates. This allows for the voltage drop between equipment and supply. Voltages in this article are the nominal supply voltages and equipment used on these systems will carry slightly lower nameplate voltages.

Power distribution system voltage is nearly sinusoidal in nature. Voltages are expressed as Root Mean Square (RMS) voltage. Voltage tolerances are for steady-state operation. Momentary heavy loads, or switching operations in the power distribution network, may cause short-term deviations out of the tolerance band. In general, power supplies derived from large networks with many sources are more stable than those supplied to an isolated community with perhaps only a single generator.

The choice of utilization voltage is due more to historical reasons than optimization of the distribution system—once a voltage is in use and equipment using this voltage is widespread, changing voltage is a drastic and expensive measure. A 230 V distribution system will use less conductor material to deliver a given amount of power because the current, and consequently the resistive loss, is lower than for a 120 V system. Incandescent light bulbs for 120 V systems are more efficient and rugged than 230 V bulbs. While large heating appliances can use smaller conductors at 230 V for the same output rating, few household appliances use anything like the full capacity of the outlet to which they are connected. Minimum wire size for hand-held or portable equipment is usually restricted by the mechanical strength of the conductors. Electrical appliances are used extensively in homes in both 230 V and 120 V system countries. National electrical codes prescribe wiring methods intended to minimize the risk of electric shock and fire.

Many areas using (nominally) 120 V make use of three-wire, single-phase 240 V systems to supply large appliances. Three-phase systems can be connected to give various combinations of voltage, suitable for use by different classes of equipment. Where both single-phase and three-phase loads are served by an electrical system, the system may be labelled with both voltages such as 120/208 or 230/400 V, to show the line-to-neutral voltage and the line-to-line voltage. Large loads are connected for the higher voltage. Other three-phase voltages, up to 830 volts, are occasionally used for special-purpose systems such as oil well pumps.

Large industrial motors (say, more than 250 HP or 150 kW) may operate on medium voltage. On 60 Hz systems a standard for medium voltage equipment is 2300/4160 V whereas 3300 V is the common standard for 50 Hz systems.

Following voltage harmonization, electricity supplies within the European Union are now nominally 230 V ± 6% at 50 Hz. For a transition period (1995–2008), countries that had previously used 220 V changed to a narrower asymmetric tolerance range of 230 V +6% -10% and those that had previously used 240 V changed to 230 V +10% -6%. Note that no change in voltage is required by either system as both 220?V and 240?V fall within the lower 230 V tolerance bands (230 V ±6%). In practice, this allows countries to continue to supply the same voltage (220 or 240?V), at least until existing supply transformers are replaced. Equipment used in these countries is designed to accept any voltage within the specified range.